Monday, 25 June 2012

Bee vs. Virus. Who decides?

One of the most frequently occurring quotes going
around referring to bees is "If the bee disappeared off the surface of
the globe then man would only have four years of life left. No more bees, no
more pollination, no more plants, no more animals, no more man," (often
attributed to Albert
Einstein). Regardless of whether or not this came from Einstein, the idea is
still one worth thinking about. Honeybees (Apis mellifera) are often the most significant pollinators
of plants, so their disappearance would be something of a concern.

Bees face many challenges. More and more people
have heard of colony collapse disorder (CCD), a phenomenon whereby colonies of
bees seem to suddenly die out. The most infuriating aspect of CCD is that the absolute
cause is unknown; whether it’s biotic factors such as viruses, chemical factors,
such as pesticides or even simply a lack of nutrition. There are many diseases
of bees, including numerous viruses, but the most significant issue bees (and their
beekeepers) face is Varroa destructor; a small mite which has caused
immense problems wherever it’s ventured.

A couple of papers have come out recently regarding
the demise of bee colonies, both concentrating on the interactions between
infestation of a colony with the Varroa mite and Deformed Wing Virus
(DWV), a small RNA virus of the family Iflaviridae.

The first study was published in Science and benefits
from studying the ‘before and after Varroa’ scenarios, as opposed to studies of
colonies in regions already infested with the mite. The scientists compared Hawaiian
islands with and without the mite, and then again once the mite had spread to
other islands. Firstly they found that over the years Varroa prevalence
increased. In turn, as Varroa levels increased, the infection levels of DWV in
each bee also increased, whilst the diversity of the DWV strains infecting the
colonies reduced. Effectively the Varroa mite had rendered a relatively resistant
population susceptible to DWV and as a result DWV exploded through the
population.

A second study, published in PLoS Pathogens, found
a similar result; as the extent of Varroa infection increased, so did the
levels of DWV. Whereas the Hawaiian study observed the situation on a grand scale,
the authors of the PLoS looked further into the interactions and found evidence
that allowing mites to feed on bee larvae resulted in an increase in DWV
levels. When they looked into the effect of the virus on the bee’s immune
system, they observed that the expression of certain genes relating to bee
immunity altered in response to infection by DWV. Essentially, bees infected
with DWV were immunosuppressed, but as a result of DWV, not Varroa feeding on
the larvae. Instead, it would appear that, by trying to repair the feeding
holes made by Varroa, the bee inadvertently weakens its resistance to DWV, thus
promoting DWV replication.

These studies add to the growing amounts of data linking
Varroa infestation and DWV as a cause of colony collapse. They also highlight
how it’s possible to view a colony as an organism itself, rather than
individuals; things such as Varroa affect the colony as a whole, and,
ultimately, it is the colony that collapses.